1. Field of the Invention
The present invention relates to an optical amplifier, and particularly, to one that changes a characteristic of controlling a current that drives an exciter laser diode, in response to the level of an optical input signal, to reduce a surge in the output of the optical amplifier when the optical input signal suddenly varies.
2. Description of the Related Art
Optical amplifiers are used for optical transmission systems. Optical power in the optical transmission system varies if an optical fiber loss fluctuates or an optical input signal suffers a short break due to maintenance or circuit switching. At this time, an erbium-doped optical fiber (EDF) serving as a part of the optical amplifier produces a surge due to induced emission.
The output of the optical amplifier involving the surge is applied to an optical-to-electrical converter composed of photosensors. The photosensors have a limit on the power of light they can accept. If the surge exceeds the rating of the photosensors, an overcurrent flows to destroy the photosensors. The optical amplifiers, therefore, must suppress such a surge output caused when an optical input signal thereto suddenly fluctuates.
Conventional optical amplifiers having an ALC (automatic level controller) have the problem of providing a surge when an optical input signal thereto involves a short break. If the surge is large, it will break elements contained in relays and terminals installed at a receiving party.